Unveiling the mysteries of Asian herbivores resource partitioning in tropical wet-grassland ecosystem

Abstract

Ecological niche partitioning is fundamental for the coexistence of sympatric species. However, the relationship between herbivore body size and forage availability on resource segregation and selection remains debatable. This study quantifies the niche differentiation and selectivity of forage species consumed by six Asian large herbivores (SLH), including mega-herbivores like Greater one-horned rhino (Rhinoceros unicornis), Asian elephant (Elephas maximus) and Asiatic wild buffalo (Bubalus arnee), and meso-herbivores like swamp deer (Rucervus duvaucelii), hog deer (Axis porcinus) and sambar (Rusa unicolor) in tropical wet-grasslands of the Brahmaputra floodplains. We hypothesized that body size influences dietary niche breadth and interspecific dietary similarities or dissimilarities, while forage availability influences forage selectivity among Asian herbivores. We used micro-histological technique, harvest method and Jacobs Index to quantify SLH foraging patterns, forage availability and preference, respectively. Results of the study revealed high niche breadth for elephant (0.56>Bs≤0.74) among mega-herbivores and for sambar (0.49>Bs≤0.64) among meso-herbivores. SLH showed a significant positive correlation between body size and niche breadth in wet season. High dietary overlap was recorded between buffalo and hog deer (0.79–0.86) and swamp deer and hog deer (0.75–0.86) among SLH, rhino and buffalo (0.80–0.83) among mega-herbivores, and swamp deer and hog deer (0.75–0.86) among meso-herbivores. High dietary dissimilarity was recorded between elephant and swamp deer (36–37 %), elephant and buffalo (27–33 %), and swamp deer and sambar (27–29 %). Compared to dry season (1354.54 ±641.30 g m˗2), significantly higher biomass was recorded in wet season (3026.93 ±1632.65 g m˗2; p<0.05). From dry to wet season, rhino, buffalo and hog deer shift their preferred forage from dicot to monocot. Dicot was the most preferred forage of elephant among mega-herbivores, and monocot of swamp deer and sambar among meso-herbivores. Mega- and meso-herbivores avoid invasives Merremia umbellata and Mimosa spp., while meso-herbivores avoid Mikania micrantha. Body size explains niche partitioning only for rhino, elephant and swamp deer. Forage availability contributed to niche breadth and forage preference. In conclusion, forage segregation, despite being challenging to interpret at taxonomic level, influences niche partitioning among mega- and meso-herbivores.